What are tuners?

Some articles on tuners, tuner:

Basic Characteristics of Made in Japan Sigma Guitars
... are three different shapes of the buttons on these tuners ... had open-backed, 3-on-a-plate non-adjustable "economy" tuners in the earliest years ... had open-back "6-on-a-plate" (non-adjustable/economy) tuners ...
Epiphone Casino - Used By The Beatles
... In the early seventies, the original tuners were replaced with a set of gold Grover tuners or machine heads ... His stripped guitar, (but still with the original nickel tuners), is first seen in the "Revolution" promo film ...
Playing The Violin - Tuning
... under the scroll, or by turning the fine tuner screws at the tailpiece ... A violin always has pegs, but fine tuners (also called fine adjusters) are optional ... Most fine tuners work by turning a small metal screw, which moves a lever that is attached to the end of the string ...